1. Field of the Invention
The present invention relates to a process for the asymmetric hydrogenation of tetra-substituted olefinic acids and derivatives thereof.
2. Description of the Prior Art
Many chemicals, particularly those having biological applications, are desirable to have in a stereochemically pure form. Quite often chemicals are prepared in a racemic form followed by optical resolution. It would be more economical and efficient to obtain the desired stereochemical form and the exclusion of the undesired stereoisomeric form(s) at an early stage of synthesis through the stereoselective creation of chiral centers.
There is a substantial amount of literature and patents on a variety of asymmetric reactions, including hydrogenation. For example, U.S. Pat. No. 3,849,480 describes the asymmetric hydrogenation of olefins with certain coordination complex catalysts. Many asymmetric hydrogenation reactions involve the treatment of ketones or highly hindered olefins. Achiwa, Fundam. Res. Homogeneous Catal., pages 549-64 (1979) discloses the asymmetric hydrogenation of amino- or acylamino olefnic compounds, such as L-beta-methylaspartic acid. However, Cullen et al., J. Amer. Chem. Soc., 102 (3), pages 988-993 states that substrates without acylamino groups are not hydrogenated by catalysts such as ferrocenylphosphine-rhodium complex catalysts. By contrast, Applicant has found that certain chiral phosphine catalyst are useful for asymmetric hydrogenation of certain highly hindered olefins, which are tetra-substituted olefinic acids and certain derivatives thereof and which do not contain the nitrogen atom one atom removed from the olefinic carbons as taught in the art.